1. Field of the Invention
The invention mainly relates to a fuel injection pipe (including a feed pipe and an injection pipe for a common-rail injection system) used in a fuel supply path of a diesel internal combustion engine.
2. Description of the Prior Art
A prior art fuel injection pipe for a diesel engine has a thick wall steel pipe 11 with a connecting head portion 12 that has a conical face 13 as shown in FIG. 1. Alternatively, the prior art fuel injection pipe may have a thick wall steel pipe 21 with a connecting head 22 having an outside arc sheet face 23, as shown in FIG. 2. The connecting head 12, 22 is molded by buckling processing using pressing pressure in the axial core direction using a punch member from the outward direction.
A steel pipe (STS370, 410 of JISG3455) of 340 N/mm2 class to 410 N/mm2 class in tensile stress generally has been used in the above-described fuel injection pipe. A cleaning technique has been developed by exhaust gas regulation of the diesel engine. The technique more perfectly sets the fuel within an engine cylinder by increasing the pressure of the fuel and injecting the fuel as fine particles for cleaning the exhaust gas. Accordingly, a high internal pressure equal to or higher than the conventional 1200 bar is loaded in the fuel injection pipe. Therefore, high internal pressure fatigue strength is required. As a countermeasure, a high tensile steel pipe of 490 N/mm2 class to 600 N/mm2 class in tensile stress tends to be used.
Such a high tensile steel pipe generally is manufactured by a drawing process. The high tensile steel pipe manufactured by the drawing process is manufactured by hot processing from an ingot, and is processed to a required size by the drawing process (pipe extension) from its thick diameter pipe. Thus, there is a case in which a fine wrinkle flaw (defect) of about 100 μm in depth is generated on the inner face of the steel pipe. It is known that this wrinkle flaw is caused by the difference in the flow of a material between the outside and the inside generated when the pipe diameter is reduced by a die from the outside of the pipe at the pipe processing time, and the pipe is rolled by a plug from the inside. Such a phenomenon is caused by the insufficiency of extension caused by approximately inverse proportion of the tension and the extension (ductility and processability). This phenomenon is generated greatly in the thick wall pipe. The inside wrinkles rolled by the plug also have small ductility. Thus, the inside wrinkles are left as flaw wrinkles. In particular, when a fine wrinkle flaw of about 100 μm in depth exists on the pipe inner face and a high internal pressure of 1200 bar to 1600 bar is repeatedly applied to the pipe interior, there is a possibility that fatigue breakdown is caused. Accordingly, the pipe may burst due to stress concentration caused in this wrinkle flaw portion.
The above wrinkle flaw on the pipe inner circumferential face is recognized as a starting point of the internal pressure fatigue breakdown and conventionally is removed by a special cutting technique. The special cutting can raise internal pressure fatigue strength. However, it was not possible to bear a pressure of about 1600 bar or more from a limit of the strength of a material. In contrast, almost no vibrational fatigue strength is almost raised. Hence, there is no effect with respect to the vibrational fatigue breakdown advanced with the outer surface as a starting point.
An autofrettage method exists for generating compression residual stress on the inner surface by applying the pressure to the pipe interior. However, in this method, there is a case in which the distribution of the residual stress is changed by subsequent elastic deformation, and is vanished. Further, when the compression residual stress is generated on the inner surface, the inner surface is processed and hardened, but the inner surface fatigue strength is insufficient approximately in the normal processing-hardening of a material. The vibrational fatigue is mainly advanced with the outer surface of the pipe as a starting point, but no strength of the outer surface is improved at all. Therefore, no vibrational fatigue characteristics were improved at all.
The present invention is made to solve such conventional problems, and an object of the present invention is to provide a high pressure fuel injection pipe excellent in internal pressure fatigue resisting characteristics, vibrational fatigue resisting characteristics and cavitation resisting property, and also excellent in sheet face flawing resisting property and bending shape stable property, and made thin and light in weight.